My prior U.S. Pat. Nos. 2,902,293 and 2,959,428, disclose tractor trailers having a pair of spaced axles to support road-engaging wheels, one of the axles (usually the front axle) being swingable about a vertical pivot to provide for steering thereof in accordance with the angular displacement of the tractor from the longitudinal centerline of the trailer.
The majority of tractor trailers seen on the roads today are what are called "tandem-tandem" trailers. In this type of trailer a pair of closely-spaced axles are supported near the rear of the trailer bed for road-engaging wheels. The front of the trailer bed is supported on the rear of the tractor frame, such frame having a pair of closely-spaced axles for road-engaging wheels.
The "tandem-tandem" trailers have many disadvantages, a serious one being that the trailer axles are spaced so close together that the legal load capacity of the trailer is reduced because the load is applied to the road surface in a small area, and also at least one of the trailer axles is non-articulated. To overcome the load disadvantage, the trailer axles were spread apart a distance of nine feet or more to comply with jurisdictional regulations, but this brought about a more serious disadvantage in that in negotiating a turn or when travelling along a curving roadway, the tires under the axles of the trailer skidded or were dragged through the turn. This required a truck trailer of greater power; caused excessive tire wear; and presented a safety hazard during road travel, since the trailer could push the tractor into a jack knife.
To overcome the disadvantage above-mentioned, the front one of the spread axles was made to swing about a vertical pivot and this axle was connected to the rear of the truck tractor by steer links. This provided a very satisfactory trailer suspension and embodiments of the construction are shown in my U.S. Pat. Nos. 2,902,293 and 2,959,428. In both of said patents, the spread axles of the trailer were connected by walking beams to equalize for bumps in the roadway and although this had some advantages, it was costly to manufacture and maintain, whereas the air bag suspension of the construction herein disclosed provides the required cushioning.
My improved construction eliminates the walking beam connection without affecting the operability of the suspension. The invention disclosed herein makes it possible to separate the two axles, and making the front axle rotatable and totally self-contained, with its own suspension and subframe. The subframe engages the undersurface of the lower flange of the two longitudinally-disposed main channels and slides therealong during turning operations to accomodate for geometrical changes caused by angular relationship of the tractor and trailer.
The construction may be easily adapted for "switch van" operation, such as is shown in my U.S. Pat. No. 3,002,636. In such cases, the subframe is elongated to connect both of the trailer axles, but the subframe still slidingly engages the main frame channels, as before. In this case, both axles move longitudinally of the main frame and the steer bars draw the whole bogie which includes one steerable axle and one fixed axle.